I have an AD V4 low band VHF up on the roof.
Not terribly useful for low band VHF as there are just a few channels using low band here in NYC.

Can I grab a decent FM signal from this rig and, if so, is there a particular splitter that will allow me to divert the FM part of the signal to my audio sys and the low band VHF to my OTA HD sys?

I realize that FM antennae are cheap enough but the V4 is already up there and I'm close enough to most of the FM transmitters that it could make sense to try it. Then again, it's not an FM rig I realize.

I'd expect the AD V4 to do quite well as an FM antenna. I'd simply use a conventional TV signal splitter. There's no reason to filter the output to the FM tuner... Any decent FM tuner will have plenty of selectivity, ability to reject TV broadcast frequencies. Connect the FM receiver the same way you connect the TVs.

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If the well is dry and you don't see rain on the horizon, you'll need to dig the hole deeper. (If the antenna can't get the job done, an amp won't fix it.)